This invention relates to electronic filters, and more particularly, to electronic filters that reject selected frequency components or frequency bands from an electronic input signal, and allow the remaining frequency components or frequency bands to freely pass to the output of the filter.
Most electrical systems include some form of an electrical filter such as a low pass, high-pass, or bandpass filter. These filters are often implemented using known combinations of resistors, inductors and/or capacitors. Typically, the filter characteristics are controlled by the particular configuration and relative values of the resistors, inductors and/or capacitors. Since resistors, inductors and capacitors are all passive components, conventional filter circuits have little or no active control over the filtering characteristics. In addition, since no active gain is provided, the values of the passive components may have to be relatively large. Having large value passive components, especially large value capacitor components, may increase the area and reduce the reliability and/or yield of the filter.
In integrated circuit technologies, capacitors are typically formed using a gate oxide type capacitor. Gate oxide capacitors include a gate oxide layer cladded by a substrate layer and a polysilicon gate layer. The capacitance value of a gate oxide capacitor is primarily dictated by the area of the polysilicon gate region. Even though the gate oxide layer is relatively thin, the amount of capacitance that can be generated per unit area is relatively small. Therefore, to generate an adequate capacitance value for many filter applications, the area of the gate oxide capacitor must be relatively large.
In many integrated circuit processes, the gate oxide layer may be susceptible to pinholing, wherein one or more pinhole defects in the gate oxide effectively short the substrate to the polysilicon gate layer. The probability of having a pinhole in any given circuit is typically dependent on the total gate oxide area in the circuit. Thus, when large gate oxide capacitors are used, the chance of having one or more pinholes in the circuit increases, and the overall reliability and/or yield of the circuit may decrease.
Therefore, it would be desirable to provide a filter that provides more active control over the filtering characteristics. It would also be desirable to provide a filter that minimizes the value of selected filter components, and in particular, capacitor components. This may help reduce the area, increase the reliability and yield, or otherwise improve the performance of the filter.
The present invention overcomes many of the disadvantages of the prior art by providing a filter that actively controls one or more of the filter characteristics. The present invention also provides a filter that helps minimize the value of selected filter components, and in particular, capacitor components. Finally, the present invention may be used to provide a filter that actively rejects a DC offset voltage or current from an input signal.
In an illustrative embodiment, a filter is provided that rejects a selected frequency component or frequency band from an input signal by actively providing an offset signal that effectively cancels out the rejected frequency component or frequency band, while allowing the remaining frequency components or frequency bands to freely pass to the output of the filter. It is contemplated that such a filter may be either a high-pass filter, a low pass filter or a band pass filter, and may be applied to either a single ended or differential input signal. In a high-pass filter, the offset signal may also be used to actively reject a DC offset voltage or current from the input signal.
For a single ended input signal, the filter may include, for example, a control circuit that provides a control signal that is related to the amplitude of the input signal. The amplitude is typically expressed as the difference between the input signal and a reference voltage, such as ground. In one embodiment, the control circuit is a buffer circuit that has a single input port for receiving the input signal. The buffer circuit can be inverting or non-inverting, depending on the application. The control circuit may alternatively be a differential amplifier circuit having a first input port and a second input port, wherein the input signal is provided to the first input port and a reference signal (e.g. ground) is provided to the second input port.
An offset circuit is also provided for receiving the control signal, and for providing an offset signal to the input terminal of the filter. The offset signal preferably effectively cancels out the input signal at the first frequency, and does not substantially effect the input signal at the second frequency. This is preferably accomplished by connecting a filter to the control signal. The filter may be provided either inside or outside of the offset circuit. In either case, the filter preferably substantially prevents the control signal from tracking the input signal at the second frequency, while substantially allowing the control signal to track the input signal at the first frequency. Alternatively, the filter may substantially prevent the control signal from tracking the input signal at the first frequency, while substantially allowing the control signal to track the input signal at the second frequency. The first frequency may be higher or lower than the second frequency.
The offset circuit may include an offset transistor, where the gate of the transistor receives the control signal. The source of the offset transistor may be coupled directly or indirectly to a reference voltage, such as VDD or ground. The drain of the offset transistor may be coupled to the input terminal of the filter. In this configuration, the control signal controls the conductivity of the offset transistors and thus the offset current supplied to the input signal.
To create a high-pass filter, the filter that is connected to the control signal may be a capacitor. The capacitor may be coupled between the control signal and ground. At low frequencies, the capacitor appears as an open, and the control signal is passed to the offset transistor relatively unencumbered. Accordingly, the offset transistor may provide an offset current that, for example, pulls the input signal high each time the input signal attempts to go low. Alternatively, and depending on the relative polarity of the control circuit and the offset transistor, the offset transistor may provide an offset current that, for example, pulls the input signal low each time the input signal attempts to go high. In either case, the input signal may remain in one state at low frequencies.
As the frequency increases beyond the high-pass pole, the capacitor begins to appear as an AC short to ground. Therefore, the control signal is substantially prevented from reaching the offset transistor, thereby removing the effect of the offset current from the filter.
It is contemplated that the capacitor may be any type of filter circuit, and may include a collection of resistors, inductors (or gyrators), and/or capacitors. Accordingly, it is contemplated that the control signal provided by the control circuit may be filtered using a low pass filter, a high-pass filter or a bandpass filter, whichever is appropriate for the particular application.
If the control circuit has gain, the capacitance required to achieve the desired high-pass pole is reduced. This may reduce the area, increase the reliability and yield, or otherwise improve the performance of the filter. For a differential input signal, the filter may include, for example, a comparator for comparing the positive and negative input signals of the differential input signal. The comparator may provide one or more control signals that are related to the difference between the positive and negative input signals. An offset circuit receives the one or more control signals, and provides one or more offset signals to the positive and negative input signals of the differential input signal. The offset signals preferably effectively remove the difference between the positive input signal and the negative input signal of the differential input signal at the first frequency, and have substantially no effect on the difference between the positive input signal and the negative input signal of the differential input signal at the second frequency. When the second frequency is higher than the first frequency, the resulting filter may be a high-pass filter. When the second frequency is lower than the first frequency, the resulting filter may be a low pass filter.
Because the offset signals preferably effectively remove the difference between the positive input signal and the negative input signal of the differential input signal at the first lower frequency, the DC offset in the differential input signal will also be actively removed by the filter.
The comparator preferably includes a differential amplifier having a positive input port, a negative input port, a positive output port and a negative output port. The offset circuit preferably includes a differential pair of transistors each having a gate. The gate of a first one of the differential pair of transistors is preferably coupled to the positive output port of the differential amplifier, and the gate of a second one of the differential pair of transistors is preferably coupled to the negative output port of the differential amplifier circuit.
The source terminals of the first and second differential pair of transistors are preferably coupled directly or indirectly to a reference voltage, such as VDD. The drain of the first one of the differential pair of transistors is preferably coupled to the positive input port of the differential amplifier. The drain of the second one of the differential pair of transistors is preferably coupled to the negative input port of the differential amplifier. In this configuration, the control signals control the offset currents supplied to the positive input port and negative input port of the differential amplifier.
To create a high-pass filter, a first capacitor may be coupled between the gate of the first one of the differential pair of transistors and ground, and a second capacitor may be coupled between the gate of the second one of the differential pair of transistors and ground. Preferably, the first capacitor and the second capacitor are matched, although this is not required.
At low frequencies, the first and second capacitors will appear as opens, and the feedback path from the outputs of the differential amplifier to the differential pair of transistors will be relatively unencumbered. Accordingly, the differential pair of transistors may provide offset currents that force the positive input port and negative input port of the differential amplifier to be substantially equal. As indicated above, this not only actively controls (e.g., eliminates) the DC offset between the positive and negative input signals of the differential input signal, but also provides a high-pass pole.
As the frequency increases beyond the high-pass pole, the first and second capacitors begin to appear as AC shorts to ground. This effectively prevents the AC control signals from reaching the gate terminals of the differential pair of transistors, thereby removing the effects of the offset current from the filter.
It is contemplated that the first and second capacitors may be any type of filter circuit, and may include a collection of resistors, inductors (or gyrators), and/or capacitors. Accordingly, it is contemplated that the control signals provided from the differential amplifier to the differential pair of transistors may be filtered using a low pass filter, a high-pass filter or a bandpass filter, as appropriate.
As indicated above, the differential amplifier preferably has significant gain. This may reduce the capacitance required to achieve the desired high-pass pole, thereby reducing the area and potentially increasing the reliability and yield of the filter. A number of methods are also contemplated including methods for filtering single ended and differential input signals. An illustrative method for filtering a single ended input signal includes the steps of: providing a control signal that is related to the difference between the input signal and a reference signal; filtering the control signal to substantially prevent the control signal from tracking the difference between the input signal and the reference signal at the second frequency, but substantially allowing the control signal to track the difference between the input signal and the reference signal at the first frequency; and providing an offset signal, controlled by the control signal, to effectively cancel out the input signal, the offset signal effectively canceling out the input signal only when the control signal substantially tracks the difference between the input signal and the reference signal.
An illustrative method for filtering a differential input signal includes the steps of: comparing the positive input signal and the negative input signal of the differential input signal; providing a control signal that is related to the difference between the positive input signal and the negative input signal; filtering the control signal to substantially prevent the control signal from tracking the difference between the positive input signal and the negative input signal at the second frequency, but substantially allowing the control signal to track the difference between the positive input signal and the negative input signal at the first frequency; and providing offset signals, controlled by the control signal, to effectively cancel out the differential input signal, the offset signals effectively canceling out the differential input signal only when the control signal substantially tracks the difference between the positive input signal and the negative input signal. Other methods are also contemplated.